The field of the invention relates to fuel vapor recovery systems coupled to internal combustion engines. In one particular aspect, the invention relates to air/fuel ratio control for engines equipped with fuel vapor recovery systems.
Fuel vapor recovery systems are commonly employed on modern motor vehicles to reduce atmospheric emissions of hydrocarbons. Typically, a storage canister containing activated charcoal is coupled to the fuel tank for adsorbing hydrocarbons which would otherwise be emitted into the atmosphere. Such storage canisters may also be utilized to capture hydrocarbons when filing the fuel tank. To cleanse the canisters, ambient air is occasionally purged through the canister for absorbing stored hydrocarbons and inducting the purged hydrocarbon vapors into the engine. In addition, fuel vapors are inducted directly from the fuel tank into the engine. In modern automobiles with fuel injected engines it has become increasingly desirable to purge fuel vapors directly from the fuel tank as often as possible. The rate of vapor flow, from both the fuel tank and canister, is typically controlled by pulse width modulating an electronically actuated solenoid valve.
In motor vehicles equipped with air/fuel ratio feedback control systems, it has been found desirable to regulate the induction of fuel vapors such that the rate of vapor flow is proportional to inducted air flow. For example, U.S. Pat. No. 4,715,340 issued to Cook et al controls the rate of vapor flow to be proportional to a calculation of inducted air flow (or, similarly, desired fuel charge calculation) such that the overall inducted mixture of air, fuel, and fuel vapor remains within the feedback system's range of authority. Air/fuel ratio transients which would otherwise occur during the onset of vapor induction are also reduced by maintaining vapor flow proportional to inducted air flow. This is accomplished by actuating the solenoid valve of the vapor recovery system with an electrical signal having a pulse width proportional to a measurement of inducted air flow.
The inventor herein has recognized several problems with conventional fuel vapor recovery systems, especially when these systems are used with engines having air/fuel ratio feedback control systems. More specifically, in turbocharged engines, supercharged engines, or multi valve per cylinder engines, there may be insufficient manifold vacuum to induct fuel vapors. Further, even when there is sufficient vacuum for vapor induction, the vacuum may not be sufficient to provide sonic vapor flow through the regulating valve. Accordingly, vapor flow through the valve will be a function of both the valve on-time and the pressure differential across the valve. Thus, maintaining fuel vapor flow as a proportion of induced air flow may not be achievable.
U.S. Pat. No. 4,530,210 issued to Yamazaki addresses only one of the problems discussed above. More specifically, in the case of a turbocharged engine, the '210 patent discloses pressurizing the fuel vapor storage canister such that vapor flow is forced into the air intake when the engine throttle is opened sufficiently to reduce intake pressure below atmospheric pressure. The inventor herein has recognized several disadvantages of the approach disclosed in the '210 patent. For example, by pressurizing the vapor canister, it may not be possible to concurrently induct fuel vapors from both the fuel tank and canister. As discussed herein above, induction of fuel vapors directly from the fuel tank has become increasingly desirable in today's fuel injected engines. Another disadvantage of the approach disclosed in the '210 patent is that vapor induction apparently cannot occur over the full engine operating cycle. As stated above, there must be a negative pressure near the engine throttle for vapor purge to occur.